A compound semiconductor is a compound that is not a single element like germanium but is obtained by combining at least two kinds of elements, and thus operated as a semiconductor. Various kinds of compound semiconductors have been developed and used in many fields. Representatively, compound semiconductors are used for light emitting devices such as LED or laser diode using photoelectric conversion effects, solar cells, and thermoelectric conversion elements using Peltier effects.
Among them, an environment-friendly solar cell not requiring any other energy source except solar light is intensively studied as an alternative energy source in the future. The solar cell is generally classified into a silicon solar cell using a single element of silicon mainly, a compound semiconductors solar cell using a compound semiconductor, and a tandem solar cell having at least two laminated solar cells with different bandgap energies.
The compound semiconductor solar cell uses a compound semiconductor in a light absorption layer that absorbs solar ray to generate an electron-hole pair. The compound semiconductor includes III-V compound semiconductors such as GaAs, InP, GaAlAs and GaInAs, II-VI compound semiconductors such as CdS, CdTe and ZnS, and I-III-VI compound semiconductors represented by CuInSe2.
The light absorption layer of the solar cell needs excellent long-term electric optical stability, high photoelectric conversion efficiency, and easy control of bandgap energy or conductivity by doping or change of composition. Also, for practical use, the light absorption layer should meet requirements in production costs and yield. Various compound semiconductors explained above do not satisfy all of these conditions, so they are suitably selected and used depending on their advantages and disadvantages.
In addition, the thermoelectric conversion element is applied to thermoelectric conversion power generation or thermoelectric conversion cooling. For example, for the thermoelectric conversion power generation, a temperature difference is applied to the thermoelectric conversion element to generate thermoelectromotive force, and then the thermoelectromotive force is used to convert thermal energy into electric energy.
Energy conversion efficiency of the thermoelectric conversion element depends on ZT value that is a performance index of thermoelectric conversion material. ZT value is determined by Seebeck coefficient, electric conductivity and thermal conductivity. In more detail, ZT value is in proportion to electric conductivity and square of Seebeck coefficient and in reverse proportion to thermal conductivity. Thus, in order to enhance the energy conversion efficiency of a thermoelectric conversion element, it is required to develop thermoelectric conversion materials with high Seebeck coefficient, high electric conductivity or low thermal conductivity.